This invention relates to display systems.
The invention is particularly, though not exclusively, concerned with head-up display systems for aircraft, that is to say, with display systems of the kind in which the display is projected onto a partially-transparent reflector in the line-of-sight of a pilot or other crew member of the aircraft so as to provide an image of the display against the background of the external scene through the aircraft windscreen. The display is normally provided in such a system by a cathode-ray tube and involves symbols that in the image seen in the partially-transparent reflector, are positioned against the background of the external scene to give indication of such factors as aircraft attitude and flight-path.
The symbols in the latter context conventionally include one or more lines that are required to be maintained superposed upon the horizon, or otherwise maintained horizontal, in the external scene viewed through the reflector, irrespective of manoeuvre of the aircraft. To this end the disposition of the one or more `horizon` lines in the reflector is varied in tilt and lateral displacement in accordance with control signals that are indicative of change of aircraft attitude in bank and pitch respectively.
There are circumstances in which it is desirable to provide the cathode-ray-tube display using a raster or other point-by-point scan, as distinct from a cursive technique in which the cathode-ray beam is caused to trace out the symbol directly. This applies especially where pictorial information as derived, for example, from a television or infra-red camera, is to be included in the display image superposed on the external scene as viewed by the pilot or other crew member. In such circumstances the one or more horizon lines and other symbology may be generated by modulation of signals in accordance with stored information relating to the mapping of the symbology in the display area, and application of these signals to the cathode-ray tube as bright-up signals appropriately synchronized to the raster or other scan.
Variation in the disposition of the symbols within the display area, and in particular the tilting and lateral displacement of the one or more horizon lines, required with manoeuvre of the aircraft, can be achieved by appropriate modification of the stored mapping. Such modification, which is performed in particular under control of signals dependent on changes in bank and pitch attitude of the aircraft, will normally necessitate the provision of substantial additional processing and storage capacity. A digital system for effecting tilting and lateral displacement of an artificial-horizon line in a cathode-ray-tube display utilizing raster scan, is described in U.S. Pat. No. 3,925,765 issued to Berwin et al on Dec. 9, 1975.
Where a raster scan is used, variation of the angle of tilt of a line in the display is usually accompanied by change in the degree of clarity or definition of the line concerned, the loss of definition being in general larger the smaller the angle of inclination from alignment with the line-scan of the raster. A staircase or notched appearance is usually experienced and slight change in the angle of tilt can readily result in disconcerting movement and even oscillatory, back-and-forth break up, of the line representation.
A significant increase in the number of line scans in the raster together with a corresponding increase in the definition with which the display symbology is pictured, would serve to reduce the visual staircase or notched effect. But there is usually in practice a standard raster to be used (for example 512- or 625-line), and an economic or space limit on the amount of information storage and processing that can be provided for picture definition. Furthermore, the signals for display of the symbology are conveniently and more economically generated using digital techniques, so the essentially discrete-element composition of the symbol representations adds to the disjointed visual effect. The display representation of each `horizon` line for example, is in essence generated by bright-up of successive elements across the cathode-ray-tube screen, and whereas these elements for an untilted line are joined up with one another in one series along one or more horizontal scan lines, the tilted-line representation is formed by disjointed series on successive, vertically-spaced scan lines of the raster.